As shown in FIG. 1, a general CD (compact disc) is formed with a thickness of 1.2 mm and a diameter of 120 mm. Also, the CD has a center hole of 15 mm in diameter, and a clamping area of 44 mm in diameter, which is formed around the center hole. The clamping area is securely placed on a turntable provided in an optical disc device by means of a clamper also provided in the optical disc device.
A data recording layer of the CD, on which data is recorded in a pit pattern, is spaced approximately 1.2 mm from the surface of the CD facing an objective lens OL of an optical pickup provided in the optical disc device. The objective lens OL, which is used in the optical pickup for the CD, has a relatively small numeral aperture NA. For example, the objective lens OL has an NA of 0.45.
As shown in FIG. 2, a general DVD (digital versatile disc) is formed with a thickness of 1.2 mm and a diameter of 120 mm. Also, the DVD has a center hole of 15 mm in diameter, and a clamping area of 44 mm in diameter, which is formed around the center hole. The clamping area is securely placed on a turntable provided in an optical disc device by means of a clamper also provided in the optical disc device.
A data recording layer of the DVD, on which data is recorded in a pit pattern, is spaced approximately 0.6 mm from the surface of the DVD facing an objective lens OL of an optical pickup provided in the optical disc device. The objective lens OL, which is used in the optical pickup for the DVD, has a relatively large numeral aperture NA. For example, the objective lens OL has an NA of 0.6.
As shown in FIG. 3, a high-density optical disc, standardization of which has been under discussion recently, such as a BD (Blu-ray disc), is formed with a thickness of 1.2 mm and a diameter of 120 mm. Also, the BD has a center hole of 15 mm in diameter, a clamping area of 33 mm in diameter, which is formed around the center hole and securely placed on a turntable provided in an optical disc device by means of a clamper also provided in the optical disc device, and a transition area of 42 mm in diameter, which is allocated between the clamping area and an information area.
A data recording layer of the BD, on which data is recorded in a pit pattern, is spaced approximately 0.1 mm from the surface of the BD facing an objective lens OL of an optical pickup provided in the optical disc device.
The objective lens of the optical pickup for the BD has the largest numeral aperture NA. For example, the objective lens has an NA of 0.85. The optical pickup for the BD uses a laser beam having a short wavelength to reproduce data recorded in a high-density pit pattern or record data in a high-density pit pattern, as compared to the optical pickup for the CD or the DVD.
Specifically, the laser beam having a short wavelength and the objective lens having a much larger numeral aperture are used in the optical pickup for the BD. Consequently, a small spot of a laser beam having an increased amount of light can be focused on the recording layer in a state in which the objective lens of the optical pickup for the BD is closer to the recording layer of the BD. Furthermore, the light transmittance distance of the laser beam having a short wavelength can be shortened, whereby property change of the laser beam and occurrence of aberration are minimized.
When a BD 10 is properly placed on a turntable 11 provided in the optical disc device, as shown in FIG. 4, a general servo operation is carried out as follows. The BD 10 is rotated at high speed by means of a spindle motor 12, a motor driver 13, and a servo controller 15. While the BD 10 is rotated, the objective lens OL of an optical pickup 14, which is also provided in the optical disc device, focuses on the BD 10 based upon a focusing servo operation, by which the objective lens OL of the optical pickup 14 is vertically moved within the range of a prescribed working distance WD. After the focusing servo operation is carried out, data recorded on the disc is normally read or data is normally recorded on the disc.
When the BD 10 is improperly placed on the turntable 11 of the optical disc device, however, for example, when the BD 10 is placed upside down on the turntable 11, as shown in FIG. 5, the recording layer of the BD 10 improperly placed on the turntable 11 is, from the objective lens OL, farther by at least 1.0 mm than that of the BD 10 properly placed on the turntable 11.
In this case, the objective lens OL cannot focus on the BD 10 simply by a normal focusing operation for moving the objective lens OL vertically although the BD 10 is rotated at high speed by means of the spindle motor 12, the motor driver 13, and the servo controller 15. Consequently, the objective lens OL is upwardly moved toward the recording layer of the BD 10 until it moves to the maximum working distance WD_Max under the control of the servo controller 15 for controlling the focusing servo operation. As a result, the objective lens OL comes into collision with the BD 10, which may cause the BD 10, the objective lens OL, or an actuator for supporting the objective lens OL to be damaged, or otherwise cause the servo operation to have fatal errors.